(ABSTRACT) Systemic lupus erythematosus (SLE, lupus) is characterized by the generation of autoantibodies that promote tissue injury. Follicular B helper T (Tfh) cells, phenotypically and functionally distinct from other T-helper (Th) cell subsets, reside in B-cell follicles and germinal centers (GCs) of secondary lymphoid organs, where they are essential for B-cell proliferation and survival, immunoglobulin (Ig) affinity selection, and development of memory B and long-lived plasma cells. The regulation of Tfh cells, and how in turn these cells promote autoreactive B cell responses, are critical to understanding the generation of pathogenic autoantibodies in SLE, and ultimately, to determining therapeutic intervention. Tfh cells, and the GC B cells they help, persist in lupus with induction of autoreactive memory B cells and autoantibody-producing plasma cells, by contrast to their appropriate waning following immunization or acute infection. The chronicity of autoreactive Tfh- and B-cell responses in lupus, integral to the disease phenotype, is not well understood. To address this phenomenon, we chronologically analyzed the phenotype and transcriptional profiles of Tfh cells in lupus. Tfh-cell cytokine production was aberrantly regulatd in murine lupus, with persistent co-production of the canonical Tfh-cell cytokine IL-21, alongside interferon-? (IFN-?). We also found simultaneous IL-21 and IFN-? double-producing Tfh cells in in the kidneys of lupus-prone mice, and in the blood of SLE patients. Among the transcriptional changes we observed in lupus Tfh cells was development of a gene signature associated with signal transducer and activator of transcription (STAT) 4, a risk allele strongly associated with the development of SLE that correlated with the maintenance and persistence of the aberrant cytokine producing Tfh cells. By contrast to immunization or acute infection in which Th-cell cytokine production resolves physiologically, in human and murine lupus it persists pathologically, with STAT4 signaling critical for maintenance. We hypothesize that the inflammatory milieu of lupus leads to STAT4-mediated transcriptional changes in Tfh cells causing them to acquire an aberrant pattern of cytokine production, which then promotes pathologic B-cell maturation with autoantibody production. Our plan in this competitive renewal application is to investigate this hypothesis, addressing how the evolving, aberrant Tfh-cell phenotype promotes autoreactive B cell maturation; assessing STAT4-mediated genomic regulation of Tfh cells in lupus; and examining the factors that promote STAT4 signaling in an effort to determine the drivers of the aberrant Tfh-cell phenotype in lupus. We will answer these questions using murine model systems, and to the extent possible, in parallel with experiments using cells from patients with SLE.